Randall RG200

[lowell]

also, just powered it up without limiter bulb. seems good however there is loud thump when it's shut off. Could that be the right side muting circuit causing it?

[J M Fahey]

Hi lowell
1) the voltages you originally measured on Q1 and Q2 were impossible, *unless* they were receiving no current at all. Looking who had to supply it (Q3), it wasn't doing so, and there were a bunch of components trying to stop it, for different particular reasons (Left side: hot amp; right side: basically overcurrent)
2) I don't think that thump is a problem.
Test that amp loud a couple days (or 1 hour) and that's enough.
Merry Xmas.

[lowell]

merry xmas. and thank you!

[lowell]

got another RG200 in w/ blown PA. I'll report on the issues, will be cool to compare this repair w/ this intial post.

[lowell]

actually this is the same amp, back in for blowing fuse. Enzo, JM, others, I'm hesistant to charge this guy another $200 to fix his amp if it's coming back in for blowing the fuse and blown output transistors. I guess I'll confirm that THAT is indeed the issue, but I'm getting 12Vdc on the speaker jack. What do you all think about having fixed this amp and it's back for the same problem? Faulty design on Randalls' part? Is it correctable? I guess it's possible my last repair wasn't perfect.

[J M Fahey]

Hi lowell.
Others may disagree, but in this particular case I'd bite the bullet and not charge the guy *again* for the repair work.
I would charge the parts only, repair it and tell him the truth: "unfortunately those transistors are being *heavily* counterfeited, (google "counterfeit transistors" and show him the page), not even the "serious" suppliers can be trusted, there's nothing "I" can do about that, I'll repair it once more for a nominal cost but you drop it off on EBay or trade-in or something like that, sorry"
Otherwise either you "burn" with him (and local customers) or you'll have to repair that lemon forever.
The design is not bad, it's very conventional, except that weird "extra" protection, but uses problematic parts.
Re-read the post from the beginning, probably you will have to do the same as before.
Good luck

[lowell]

Ok so amp's back up and running again. There is a pop on turn-on and turn-off. I know this comes w/ the territory in ss amps. My final concern w/ this amp is the bias. Enzo mentioned that R47 may control this. The big question for me is: How do you know what current to bias solid state power transistors to? Does it depend on the device? V+/V-? Output class? Is measuring at the voltage across the driver bases a way of biasing? My initial thought would be to bias the power transistors bases to .7v, so if I'm correct in guessing that, I'd think the bias servo controls this.

[J M Fahey]

It's not the same as in tubes.
Here we only want a little current (a few mA) through the output transistors to ensure that they do not clip/kill low level signals; crossover distortion is UGLY.
This amp uses a salad of technologies: a diode D2, a plain Jane resistor R47 and "a saturated transistor with a resistor in series" or Q6, "an emitter-base diode with a resistor in parallel", take your pick. It looks like the designer was not too sure about what to do and threw all spices at once into the pot.
The bias adjustment element is, as Enzo said, R47, and there's a note referred to it in the schematic, unfortunately in Korean or Chinese.
Is it a preset or trimmer pot or a fixed, factory selected resistor?
Anyway, measure the emitter to emitter voltage drop across R25+R26 or R29+R30.
I guess any value under 30 or 40 mV is fine (around 50mA tops per device).
In a troublesome amp, better cold biased than the opposite.
Good luck.
PS: I *think* D2 and Q6 are in thermal contact with the heat sink, are they?

[lowell]

Q6 yes but not D2. I get no voltage reading from R25+R26 emitters or R29+R30 emitters??

[J M Fahey]

Amp ON, no signal, everything on "0", no speaker load.
1) Multimeter on 200mV DC scale. Red to a C5198 emitter, Black to an A1491 emitter, ¿what do you read?
2) Multimeter on 2V DC scale. Black to union of R25/R26, Red to its base, ¿reading?
3) Same as before, but Red to base of A1941 , ¿reading?
You might be slightly underbiased.
Personally I much prefer that situation on an abused-by-definition musical instrumet amplifier.
It is unbearable on a Hi Fi amp, when listening to piano or String Quartet music at home, late at night, but in an environmet where the noise floor is a drummer ......

[lowell]

1) Multimeter on 200mV DC scale. Red to a C5198 emitter, Black to an A1491 emitter, ¿what do you read?
0v
2) Multimeter on 2V DC scale. Black to union of R25/R26, Red to its base, ¿reading?
Black to union of R25/R26, Red to 1941 base:
400mv
3) Same as before, but Red to base of A1941 , ¿reading?
not sure what you mean here... where's black lead?

[J M Fahey]

Hi Lowell.

where's black lead?

As stated on the beginning of the phrase, "same as before", meaning still "Black to union of R25/R26"
Anyway I now know that your amp is underbiased, .... but keep it as-is.
It was born that way and nobody ever complained about that.
If it burns again they *will* complain, so do not "fix" what was not a problem for the user.
I would add a 75ºC to 85ºC cutoff bolted to the heatsink, I don't trust the fancy protection circuit they kludged.
I'd also add an internal, "unseen to the musician" fuse , the next size up as the "official" one.
Say: official one 4A; hidden one 5A. So if they replace the external one with a nail or a piece of crumpled aluminum paper, you are still limiting the damage .

[lowell]

Ok thanks JM. What is a 75ºC to 85ºC cutoff?

[J M Fahey]

a 75ºC to 85ºC cutoff

is one of those thermal cutoffs or breakers that most (specially Peavey) uses, attached to heatsinks and in extreme cases (Peavey again) in direct contact with a TO3 transistor case.
Its normally closed contacts are usually in series with one of the line power wires so in practice it "pulls the plug" when hot.
Look at any SS Peavey schematic.
A 75 to 85 Degree Centigrade (ºC) rated will be fine.
I'm sure some caritative soul will ppst some picture or even better, provide a link to some supplier, with pictures or drawings. (WOW)